14,870 research outputs found
Developmental design, fabrication, and test of acoustic suppressors for fans of high bypass turbofan engines
An analysis procedure was developed for design of acoustically treated nacelles for high bypass turbofan engines. The plan was applied to the conceptual design of a nacelle for the quiet engine typical of a 707/DC-8 airplane installation. The resultant design was modified to a test nacelle design for the NASA Lewis quiet fan. The acoustic design goal was a 10 db reduction in effective perceived fan noise levels during takoff and approach. Detailed nacelle designs were subsequently developed for both the quiet engine and the quiet fan. The acoustic design goal for each nacelle was 15 db reductions in perceived fan noise levels from the inlet and fan duct. Acoustically treated nacelles were fabricated for the quiet engine and quiet fan for testing. Performance of selected inlet and fan duct lining configurations was experimentally evaluated in a flow duct. Results of the tests show that the linings perform as designed
Observational evidence for stochastic biasing
We show that the galaxy density in the Las Campanas Redshift Survey (LCRS)
cannot be perfectly correlated with the underlying mass distribution since
various galaxy subpopulations are not perfectly correlated with each other,
even taking shot noise into account. This rules out the hypothesis of simple
linear biasing, and suggests that the recently proposed stochastic biasing
framework is necessary for modeling actual data.Comment: 4 pages, with 2 figures included. Minor revisions to match accepted
ApJL version. Links and color fig at
http://www.sns.ias.edu/~max/r_frames.html or from [email protected]
The Environment of ``E+A'' Galaxies
The violent star formation history of ``E+A'' galaxies and their detection
almost exclusively in distant clusters is frequently used to link them to the
``Butcher-Oemler effect'' and to argue that cluster environment influences
galaxy evolution. From 11113 spectra in the Las Campanas Redshift Survey, we
have obtained a unique sample of 21 nearby ``E+A" galaxies. Surprisingly, a
large fraction (about 75%) of these ``E+A''s lie in the field. Therefore,
interactions with the cluster environment, in the form of the ICM or cluster
potential, are not essential for ``E+A'' formation. If one mechanism is
responsible for ``E+A''s, their existence in the field and the tidal features
in at least 5 of the 21 argue that galaxy-galaxy interactions and mergers are
that mechanism. The most likely environments for such interactions are poor
groups, which have lower velocity dispersions than clusters and higher galaxy
densities than the field. In hierarchical models, groups fall into clusters in
greater numbers at intermediate redshifts than they do today. Thus, the
Butcher-Oemler effect may reflect the typical evolution of galaxies in groups
and in the field rather than the influence of clusters on star formation in
galaxies. This abstract is abridged.Comment: 39 uuencoded, compressed pages (except Fig 1), complete preprint at
ftp://ociw.edu/pub/aiz/eplusa.ps, ApJ, submitte
Physical Bias of Galaxies From Large-Scale Hydrodynamic Simulations
We analyze a new large-scale (Mpc) numerical hydrodynamic
simulation of the popular CDM cosmological model, including in our
treatment dark matter, gas and star-formation, on the basis of standard
physical processes. The method, applied with a numerical resolution of
kpc (which is still quite coarse for following individual galaxies,
especially in dense regions), attempts to estimate where and when galaxies
form. We then compare the smoothed galaxy distribution with the smoothed mass
distribution to determine the "bias" defined as on scales large compared with the code
numerical resolution (on the basis of resolution tests given in the appendix of
this paper). We find that (holding all variables constant except the quoted
one) bias increases with decreasing scale, with increasing galactic age or
metallicity and with increasing redshift of observations. At the Mpc
fiducial comoving scale bias (for bright regions) is 1.35 at reaching to
3.6 at , both numbers being consistent with extant observations. We also
find that Mpc voids in the distribution of luminous objects are
as observed (i.e., observed voids are not an argument against CDM-like models)
and finally that the younger systems should show a colder Hubble flow than do
the early type galaxies (a testable proposition). Surprisingly, little
evolution is found in the amplitude of the smoothed galaxy-galaxy correlation
function (as a function of {\it comoving} separation). Testing this prediction
vs observations will allow a comparison between this work and that of Kauffmann
et al which is based on a different physical modelingmethod.Comment: in press, ApJ, 26 latex pages plus 7 fig
Direct constraints on the dark matter self-interaction cross-section from the merging galaxy cluster 1E0657-56
We compare new maps of the hot gas, dark matter, and galaxies for 1E0657-56,
a cluster with a rare, high-velocity merger occurring nearly in the plane of
the sky. The X-ray observations reveal a bullet-like gas subcluster just
exiting the collision site. A prominent bow shock gives an estimate of the
subcluster velocity, 4500 km/s, which lies mostly in the plane of the sky. The
optical image shows that the gas lags behind the subcluster galaxies. The
weak-lensing mass map reveals a dark matter clump lying ahead of the
collisional gas bullet, but coincident with the effectively collisionless
galaxies. From these observations, one can directly estimate the cross-section
of the dark matter self-interaction. That the dark matter is not fluid-like is
seen directly in the X-ray -- lensing mass overlay; more quantitative limits
can be derived from three simple independent arguments. The most sensitive
constraint, sigma/m<1 cm^2/g, comes from the consistency of the subcluster
mass-to-light ratio with the main cluster (and universal) value, which rules
out a significant mass loss due to dark matter particle collisions. This limit
excludes most of the 0.5-5 cm^2/g interval proposed to explain the flat mass
profiles in galaxies. Our result is only an order-of-magnitude estimate which
involves a number of simplifying, but always conservative, assumptions;
stronger constraints may be derived using hydrodynamic simulations of this
cluster.Comment: Text clarified; some numbers changed slightly for consistency with
final version of the accompanying lensing paper. 6 pages, uses emulateapj.
ApJ in pres
A cosmological model in Weyl-Cartan spacetime
We present a cosmological model for early stages of the universe on the basis
of a Weyl-Cartan spacetime. In this model, torsion and
nonmetricity are proportional to the vacuum polarization.
Extending earlier work of one of us (RT), we discuss the behavior of the cosmic
scale factor and the Weyl 1-form in detail. We show how our model fits into the
more general framework of metric-affine gravity (MAG).Comment: 19 pages, 5 figures, typos corrected, uses IOP style fil
Morphological Variation, Karyology, and Systematic Relationships of \u3ci\u3eHeteromys gaumeri\u3c/i\u3e (Rodentia: Heteromyidae)
Morphological variation was assessed within and among populations of Heteromys gaumeri using univariate and multivariate statistical analyses of external and cranial measurements. Although patterns and amount of nongeographic variation in H. gaumeri were similar to other heteromyines, geographic variation was relatively conservative. Mean values of most characters were statistically homogeneous among localities and spatially unpatterned. Consequently, no association was found between levels of within- and among-sample variation for individual characters (the Kluge-Kerfoot phenomenon ). Populations of H. gaumeri were chromosomally monomorphic. The lack of morphological and chromosomal variation in H. gaumeri contrasts sharply with patterns in other heteromyines. Heteromys gaumeri is morphologically and chromosomally distinct from the H. desmarestianus species group (to which it is currently assigned) and appears to share some primitive characters with Liomys (the sister group of Heteromys). We recommend that H. gaumeri be removed from the H. desmarestianus group.
Spanish abstract:
La variación morfológica intra e interpoblacional de Heteromys gaumeri fue evaluada usando análisis estadísticos univariados y multivariados de medidas externas y craneales. A pesar de que los patrones y cantidad de variación intrapoblacional en H. gaumeri fue similar a la de otros heterominos, la variación geográfica fue relativamente conservadora. Los valores promedio de la mayoría de los caracteres fueron estadisticamente homogeneos entre las localidades, sin mostrar ningún patrón de variación espacial. En conservencia, no se encontró asociación alguna entre los niveles de variación intra e interpoblacional para caracteres individuates ( fenómena Kluge-Kerfoot ), Las poblaciones de H. gaumeri fueron monomórficas cromosómicamente. La falta de variacion tanto morfológica como cronosómica en H. gaumeri contrasta marcadamente con los patrones encontrados anteriormente para otros heteróminos. Heteromys gaumeri es morfológica y cromosómicamente distinguible del grupo H. desmarestianus (al cual se asigna actualmente) y aparentemente comparte algunos caracteres primitives con Liomys (el grupo hermano de Heteromys). Nosotros recomendamos que se remueva a H. gaumeri del grupo H. desmarestianus.
Portuguese abstract:
Avalia-se a variação morfológica intra- e interpopulacional de Heteromys gaumeri, através de análises estatisticas uni- e multivariadas de medidas externas e craniais. Apesar dos padrões, e da quantidade de variação intrapopulacional em H. gaumeri serem similares aos de outros heteromídeos, a variação geográfica é relativamente conservadora. Os valores médios da maior parte dos caráteres examinados são estatìsticamente homogeneos entre as localidades, e não surgiu nenhum padrão de variações locais. Consequentemente, não foram encontradas assoçiacões entre os níveis de varaiações intra- e interpopulacionais para caráteres individuais (o “fenômeno Kluge-Kerfoot”). Populações de H. gaumeri mostraram-se cromossômicamente monomórficas. A falta de variação morfológica ou cromossômica em H. gaumeri é altamente contrastante aos padrões encontrados em outros heteromídeos. Heteromys gaumeri distinguese tanto morfológica quanto cromossômicamente do grupo H. desmarestianus, ao qual está atualmente designado, e aparentemente possue caráteres primitivos em comum com Liomys—grupo irmão de Heteromys. Recomendamos que H. gaumeri seja removido do grupo H. desmarestianus
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